Don't widen in lubs

Widen in lubs only if

 - the type is unstable, or
 - we are after erasure

This need to be complemented by performance work as otherwise complexity of
subtype checking will explode.

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1530,8 +1530,10 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
         val t2 = mergeIfSuper(tp2, tp1, canConstrain)
         if (t2.exists) t2
         else {
-          val tp1w = tp1.widen
-          val tp2w = tp2.widen
+          def widen(tp: Type) =
+            if (ctx.scala2Mode || ctx.erasedTypes) tp.widen else tp.widenIfUnstable
+          val tp1w = widen(tp1)
+          val tp2w = widen(tp2)
           if ((tp1 ne tp1w) || (tp2 ne tp2w)) lub(tp1w, tp2w)
           else orType(tp1w, tp2w) // no need to check subtypes again
         }

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -827,11 +827,6 @@ trait Checking {
     }
     else tpt
 
-  /** Check that `tpt` does not refer to a singleton type */
-  def checkNotSingleton(tpt: Tree, where: String)(implicit ctx: Context): Tree =
-    if (tpt.tpe.isSingleton) errorTree(tpt, ex"Singleton type ${tpt.tpe} is not allowed $where")
-    else tpt
-
   /** Verify classes extending AnyVal meet the requirements */
   def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context): Unit =
     Checking.checkDerivedValueClass(clazz, stats)
@@ -1048,7 +1043,6 @@ trait NoChecking extends ReChecking {
   override def checkNoDoubleDeclaration(cls: Symbol)(implicit ctx: Context): Unit = ()
   override def checkParentCall(call: Tree, caller: ClassSymbol)(implicit ctx: Context): Unit = ()
   override def checkSimpleKinded(tpt: Tree)(implicit ctx: Context): Tree = tpt
-  override def checkNotSingleton(tpt: Tree, where: String)(implicit ctx: Context): Tree = tpt
   override def checkDerivedValueClass(clazz: Symbol, stats: List[Tree])(implicit ctx: Context): Unit = ()
   override def checkTraitInheritance(parentSym: Symbol, cls: ClassSymbol, pos: SourcePosition)(implicit ctx: Context): Unit = ()
   override def checkCaseInheritance(parentSym: Symbol, caseCls: ClassSymbol, pos: SourcePosition)(implicit ctx: Context): Unit = ()

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1361,7 +1361,7 @@ class Typer extends Namer
           checkSimpleKinded(checkNoWildcard(arg)))
       else if (tpt1.symbol == defn.orType)
         checkedArgs = checkedArgs.mapconserve(arg =>
-          checkNotSingleton(checkSimpleKinded(checkNoWildcard(arg)), "in a union type"))
+          checkSimpleKinded(checkNoWildcard(arg)))
       assignType(cpy.AppliedTypeTree(tree)(tpt1, checkedArgs), tpt1, checkedArgs)
     }
   }

tests/pos/single-unions.scala

@@ -0,0 +1,5 @@
+object A {
+  val x1: 3 | 4 = 3
+  val x2: 3 | 4 = 4
+  val x3: 3 | 4 = if (???) 3 else 4
+}